1. Field of the Invention
The present invention relates to an image forming apparatus having a sheet feeding device that feeds sheets stacked on a sheet stacking portion while separating the sheets from the uppermost sheets being stacked on the sheet stacking portion.
2. Description of the Related Art
In the related art, a sheet feeding device is incorporated into an image forming apparatus that forms images on sheets by an electrophotographic process, for example, so as to supply the sheets one by one to an image forming portion that forms images on the sheets. For example, the sheet feeding device includes a sheet cassette in which sheets are accommodated, a feeding roller that feeds sheets from the sheet cassette, and a friction separation portion that is provided pressure contacted with the feeding roller. When the uppermost sheets being fed by the feeding roller are conveyed, there is a case where the next sheet disposed thereunder is fed in an accompanied manner (which will be referred to as an accompanied feeding). In such a case, the uppermost sheets are separated into one sheet at a separation nip between the feeding roller and the friction separation portion and conveyed.
In this configuration, when the sheets are fed continuously in a state where the feeding roller is pressure contacted with the sheets, the next sheet which is fed in an accompanied manner with the sheet being fed will be continuously fed in the accompanied manner and pass through the separation nip, thus causing a multiple feeding. In the related art, this multiple feeding problem was solved by releasing the pressure-contact between the feeding roller and the sheet during the period when the sheets are conveyed by the feeding roller, thus preventing the accompanied feeding of the next sheet.
A sheet feeding device is known having a configuration in which the pressure-contact between the feeding roller and the sheet is released during the sheet feeding operation. According to this sheet feeding device, a pressing plate which is a sheet stacking portion having sheets placed thereon is supported to be pivotable upward and downward about a pivot shaft. The pressing plate is pivoted upward by being urged by a pressing plate spring. When the pressing plate is pivoted upward, the uppermost sheet comes into contact with the feeding roller, and the sheet is fed by rotation of the feeding roller. During the period when the sheet is being fed, the pressing plate is depressed by a pressing plate releasing cam being rotated by driving of a motor and is separated from the feeding roller. In this way, it is possible to prevent the accompanied feeding of the next sheet subsequent to the sheet being fed by the feeding roller and suppress a multiple feeding of sheets. This technique is described in Japanese Patent Application Laid-Open No. H11-301864.
However, when the sheet being fed are thin sheets (for example, having a basis weight of 75 g/m2 or smaller), the leading end of the next sheet being fed in the accompanied manner will be folded and/or rolled by coming into contact with a conveyance guide in front of the separation nip. When the timing of separating the feeding roller from the sheet is accelerated to comply with the feeding of the thin sheet, a thick sheet (for example, having a basis weight of 105 g/m2 or more) is not easily caught at the separation nip since the sheet is thick and rigid. Thus, the thick sheet will not be fed properly. Particularly, this phenomenon will become prominent as the sheet becomes thicker and more rigid.